1. Field of the Invention
The invention relates to a USB charging method, and in particular to a USB charging method having protection function.
2. Description of Prior Art
Since USB interface comprises plug-and-play characteristic and can output stable power (for example, USB 2.0 interface can provide 5V/0.5 A power), many electronic devices with lower power consumption, such as smart devices, MP3 players, set-top boxes, etc., use USB connector as substitute for traditional DC-Jack for power transmission.
Recently, USB Implementers Forum (USB IF) released a new USB interface named USB Type-C for USB 3.1 standard. Also, USB IF further released a new power transmission standard named USB Power Delivery (USB PD) for providing enough power for electronic devices with higher power consumption, such as Tablet PCs, laptop computers, PC's monitors, etc., through USB interface.
A USB Type-C connector compliant with USB PD standard can support output voltages of multi-stages, for example, 5V, 12V, 20V, etc. Further, with adjustable output currents, the USB Type-C connector can output power up to 100 W (20V/5 Å).
In view of the convenience of the USB PD standard, some electronic devices demanding higher power (such as laptop computers) are now using the USB Type-C connector to transmit power. However, if the firmware of these electronic devices are not optimized according to USB PD standard's characteristics, powering electronic source devices may output unsuitable power that is unsuitable to these electronic sink devices, and may cause damage.
Specifically, after receiving a power requirement ACK from a electronic source device (such as a power supply), an electronic sink device's firmware may inherently request the largest power provided by the electronic source device as a best solution for completing charging as soon as possible. However, the largest output voltage that the USB Type-C connector can support under USB PD standard is 20V, which may exceed a best working voltage that the electronic sink device can endure. In this scenario, if the firmware of the electronic sink device inherently request the largest power offered by the electronic source device after receiving the power requirement ACK, the electronic sink device may be burnt out.
Furthermore, most electronic sink devices use a power supply or a battery as power source, and have acceptable voltage upper limit settings for battery being larger than for power supply (ie. the electronic sink device would decrease output current received from the battery for stabilization). If the firmware of the electronic sink device is not optimized and is unable to recognize whether it is the power supply or the battery that sent the power requirement ACK, it may request an unsuitable output power and result in errors during its charging procedure.